A semiconductor device can include semiconductor elements, such as Metal Oxide Semiconductor Field Effect Transistors (MOSFETs), integrated on a single circuit chip, the semiconductor elements with different tolerance of gate voltage, that is, with different gate breakdown voltages. As a wafer processing process for manufacturing such a semiconductor device, a series of steps of forming gate oxide films with different thicknesses onto a single silicon substrate can be applied.
Conventional steps of forming the gate oxide films with different thicknesses onto the single silicon substrate cause a LOCal Oxidation of Silicon (LOCOS) film adjacent to the gate oxide films to have a small thickness, thus affecting transistor properties. To address this problem, proposed is a method of forming a nitride film as a protective film, like techniques in, for instance, Patent Documents 1 to 4.